1. Field of the Invention
The present invention pertains to photorefractive materials that are used as holographic storage materials and, particularly, to methods and materials for optical fixation of information in the photorefractive materials for long term storage where the information can be read out non-destructively with a uniform beam, archived for long term storage, and erased if desired.
2. Description of Prior-Art
In volume data storage, information is stored in the form of holograms throughout the three dimensional volume of a light sensitive material. Volume data storage offers great potential for extremely dense mass storage and fast information processing. For example, it is possible to store a terabyte or more of data on a one-centimeter cube.
Photorefractive materials, such as ferroelectric materials, are most suitable for read and write or read-only volume memories. The term xe2x80x9cphotorefractive effectxe2x80x9d refers to changes in the index of refraction of an electro-optic material when the material is illuminated with non-uniform light. The index of refraction is defined as the ratio of the speed of light in a vacuum to that in a material.
Photorefractive materials possess suitable traps that are partly occupied by photosensitive electric charges. The traps originate in the material as a result of natural defects during crystal growth. These defects can occur whether the crystal is doped or undoped. Traps can be increased or enhanced by doping. These defects are mechanical flaws in the lattice structure, e.g., through the lattice structure missing atoms at some lattice sites.
A great variety of photorefractive materials is known to exist including organic and inorganic materials. For example, Rakuklujic and Yarviv, Photorefractive materials for optical computing and image processing, SPIE Vol. 881 Optical Computing and Nonlinear Materials (1988) describes a number of well known materials including BaTiO3, SBN, BSKNN, LiNbO3, KNbO3, BSO, GaAs, and InP, which all demonstrate photorefractive effects that may also be enhanced by doping. The Rakuklujic et al. article mathematically defines terms including steady-state change in the refractive index, response time, and photorefractive sensitivity. The article provides a plurality of material parameters for each material, in both doped and undoped form. These parameters include data corresponding to the wavelength of light that induces a photorefractive effect in each material. The refractive index and photorefractive response time are listed for many materials, e.g., the materials can be selected to vary the photorefractive response time from one second to 100 picoseconds.
The family of tungsten bronze structures has been studies for its photorefractive effect, as reported in Neurgasonkaur et al, Development and modification of photorefractive properties in the tungsten bronze family crystals, 26 Optical Engineering No. 5 pp. 392-405 (May 1987), as well as in R. R. Neurogaonkar et al., Photorefractive tungsten bronze materials and applications, SPIE Vol. 1148 Nonlinear Optical Properties of Materials (1989).
As shown by the aforementioned articles, dopants may be added to the crystal melt to grow crystals having relatively more or less defects than would form during the growth of an undoped crystal. A dopant is typically an atom having a different oxidation state, ionic radius, or affinity for surrounding materials, than the other atoms that are normally found in the lattice. In practice, dopants can insert themselves into the lattice structure to either create or compensate defects, or dopants can decorate the surface of polycrystalline grains. Photorefractive materials are usually doped with a single or multiple dopant species to improve their refractive properties. For instance, lithium niobate is usually doped with iron, of various oxidation states to increase the sensitivity of the material to light. Other dopants that have been used to enhance the photorefractive effect include Ce, Cr, and Mn in various valence states, though an oxidation state of +3 is often preferred. Other dopants of similar oxidation states and which are similarly situated on the periodic table can also be used.
The Electro-optic Effectxe2x80x94Trap-charge Theory of Refractance
Holograms are written into photorefractive materials by the action of light on these materials according to the well-known photorefractive effect. The action of light of certain wavelengths and/or activation energy over time produces local field distortions that are associated with an increase or decrease in refraction. The localized differences in refraction are capable of storing data in the form of a trapped image. Two significant problems in the art include fading of the trapped image with time by dissipation of the image during storage or fading of the image during readout. The nonlimiting discussion below provides a generalized theory of why these two image fading phenomenon occur. Other image fading mechanisms may also play a role in the fading phenomenon. Only one technique, namely, that of thermal fixation, has been developed to overcome the problem of image fading during readout. No techniques have been developed to overcome the problem of image dissipation over time in storage conditions. Failure to overcome the problem of image dissipation during storage conditions precludes the use of holograms for long term archival storage.
In a photorefractive material, photosensitive charges move within the material under the influence of light. FIG. 1 illustrates a photorefractive material 100 having a plurality of trapped photosensitive electric charges 102 and 104, as well as the motion of these charges when they are excited by light 106. When the photorefractive material 100 resides in darkness, the photosensitive electric charges 102 and 104 remain where they are originally located. Illumination of the photorefractive material 100 by light 106 excites the photosensitive electric charges 102 and 104 to a mobile state 108 causing the charges to leave their original locations and migrate through the crystal by one or more charge transfer mechanisms 110 and 112.
The charge transfer mechanisms 110 and 112 are often referred to in the art as drift 110 and diffusion 112. Drift 110 occurs when photosensitive charges 102 and 104 move under the action of a static electric field that is applied to the material. Diffusion occurs because the photosensitive charges 102 and 104 tend to move from regions of high light intensity to regions of low light intensity. The photosensitive electric charges 102 and 104 migrate through the crystal structure by these mechanisms until they are eventually retrapped, e.g., as at site 114 following a relaxation of energy along pathway 116.
Excitation of trapped charges 102 and 104 by light 106 causes the charges to move away from the light 106 by drift 110 and diffusion 112 until they are retrapped at other sites, e.g. site 114. When a photosensitive electric charge, e.g., charge 102 or 104, migrates, it leaves behind an immobile ionized trap 118, e.g., trap 118. This trap creates a space charge electric field, which distorts the material lattice and, consequently, modulates the index of refraction of the material via the electro-optic effect, as explained in more detail below.
FIG. 2 shows two coherent light beams 200 and 202, which intersect across a three dimensional region 204 within a photorefractive material to create a spatially periodic light interference pattern, as shown in interference pattern 300 of FIG. 3. The periodic light interference pattern 300 excites photosensitive charges in the material, which migrate away from the light by diffusion and drift in the manner described with respect to FIG. 1. The motion of the charges disturbs the charge equilibrium that was present in the material before illumination. This disturbance sets up a corresponding electronic charge distribution 302, which, in turn, creates a corresponding periodic space charge electric field 304 within the material. In turn, the space charge electric field 304 modulates the index of refraction of the material via the electro-optic effect. The refractive index change modulation 306 is called an index grating and constitutes a hologram. The electric field 304 and the index of refraction 306 have the same periodicity as the light pattern, but are shifted in phase.
The prior art process of charge migration that results in the index of refraction 306 is a reversible one. Long term thermodynamic stability in the crystal favors an evenly distributed space charge that reverses the photorefractive effect in prior art crystals. When the photorefractive material is left in the dark, the changes forming the index of refraction 306 subside slowly to a uniform distribution corresponding to the initial state of the photorefractive material. The hologram thus persists in the photorefractive material until the changes in the index of refraction 306 disappear and the charge equilibrium is substantially restored in the material. The period of time that the hologram remains in the material in the dark is a function of the material""s dielectric constant and is known as the material dark storage time. The dark storage time varies from milliseconds to a few months depending on the type of photorefractive material.
The index of refraction 306 is read by illuminating the photorefractive material with a uniform light beam to reproduce the stored image. This process of reading, according to the prior art, accelerates degradation of the stored image because the applied light evenly redistributes the photoexcited charges to cancel the space charge electric field 304. Accordingly, the changes in the index of refraction 306 vanish, and the hologram or index of refraction 306 to erases. The rate of hologram erasure in a photorefractive material when it is illuminated with a uniform light beam depends on the parameters of the material, the intensity and wavelength of the uniform beam, and the strength of the recorded hologram. Generally, this phenomenon of hologram erasure upon image readout is a tremendous problem in the art.
Conventional Read/Write Technologies
As discussed above, information consisting of a bit array or image is recorded in a photorefractive material in the form of holograms. Holograms are recorded in a photorefractive material by intersecting two coherent light beams, e.g., e.g., beams 200 and 202, where one beam is known as the object beam 200 and the other beam is known as the reference beam 204. The object beam 200 carries information while the reference beam 202 represents a spatial address that used to reference the information for readout from the photorefractive material. The information is often encoded in the object beam 200 using a page composer, which is a modulating device such as an electrically or optically addressed spatial light modulator, which encodes the incoming object beam with a light pattern.
A hologram recorded in a photorefractive material is reconstructed by blocking the object beam 200 and illuminating the material with the reference beam 202. When the reference beam 202 impinges on the photorefractive material, some of the light is deflected by the index grating in the direction of the object beam. Two beams emerge from the material, i.e. the reference beam and a replica of the object beam. The replica of the object beam is known as the diffracted beam and represents the reconstructed hologram. The reconstructed hologram, which is usually intercepted by a detector array or a charge coupled device for further processing, is short lived, according to prior art materials and practices. Its lifetime is limited to few""seconds or minutes depending on the material.
There are various known methods to record multiple holograms in a photorefractive material and read them out selectively. These methods are known as angular addressing, wavelength multiplexing, and light modulation addressing. In angular addressing, the angle that is formed by the intersection of the object beam and the reference beam during recording serves as the hologram address. Multiple holograms can thus be recorded in the storage material by varying the angle between the object and reference beam. A particular hologram is read from the photorefractive material by blocking the object beam 200 and illuminating the photorefractive material in the angular position that was assigned to the hologram during recording.
In wavelength multiplexing, multiple holograms are recorded in a storage material with reference beams that are plane waves of different wavelengths. Each wavelength corresponds to a hologram address. Stored holograms are read from the photorefractive material by illuminating the material with the plane wave reference beam of the specific wavelength that is assigned to it during its recording.
In light modulation addressing, the reference beam is encoded with a light pattern, which constitutes a hologram address. Different light patterns are assigned to different holograms. A transparency or a light modulating device such as an electrically or optically addressed spatial light modulator encodes the reference beam with an address pattern. During hologram readout, the encoded reference beam illuminates the storage material.
Read-out Erasure and Dark Storage
The main drawbacks of volume holographic storage in photorefractive materials is erasure of holograms as they are read out from storage and gradual dissipation of the stored image that naturally occurs even when the storage material is maintained in total darkness. The read-out erasure problem occurs by the action of light to produce an additional photorefractive effect that destroys the stored image as the trapped interference pattern is being read-out.
Efforts to remedy this problem have resulted in various techniques to preserve the information stored in photorefractive materials. For instance, heating the photorefractive material to high temperatures creates an ionic replica of the hologram recorded in the material. The ionic replica of the hologram persists against uniform light illumination and solves the read-out erasure problem, but the stored image still continues to degrade during storage, e.g., even during storage in the dark. Even the best thermally preserved images tend to exhibit complete or substantial degradation after about five or six months of dark storage. When the material is reheated to a higher temperature than the temperature used for fixing, the ionic replica vanishes.
Holograms are also fixed when a high voltage is applied across a photorefractive material. This electrical fixation forms a replica of the hologram that is recorded in the photorefractive material. The replica of the hologram is stable for many hours against uniform illumination by the reference beam. The replica of the hologram erases when an external field is applied again across the photorefractive material during uniform illumination.
A third technique that yields non-destructive hologram readout requires that the wavelength used for writing the hologram be different than that used for reading it out. The material is usually less sensitive to the wavelength used for reading out the hologram. Another technique uses the same wavelength for writing and reading out the hologram but requires that the polarization of the recording beam be orthogonal to that of the readout beam. Techniques that are used to refresh the information, e.g., as by recalling and rewriting the stored image, can also be used to counter hologram erasure in a photorefractive material.
The conventional fixing methods that are described above have not yet been applied to commercial devices because they present several drawbacks for commercial storage systems. Some of the limitations of each of these techniques are outlined below:
a. Thermal fixing requires a heating surface such as an oven to bring the material to high temperatures, which are very impractical because commercial storage systems must be small, fast, compact, and safe;
b. Electrical fixation requires large power supplies to achieve the voltage levels necessary for hologram fixation;
c. Information loss occurs when different wavelengths are used, and two different lasers are required, which renders the system complex and expensive;
d. When a single laser is used to read and write data, a device that switches the polarization of the reference beam during readout is required, which makes the system speed dependent on the speed of the switching device;
e. When different polarization""s are used for hologram recording and readout, information can only be read out for few hours, and two lasers are required, which makes the system complex and expensive;
f. System refresh techniques result in information loss and require a complex system design while these techniques do not fix the data permanently and, while they recreate a replica of the hologram every time the hologram is read out, the quality of the image degrades with each refreshed writing; and
g. None of the technologies that have been developed to date overcome the problem of image dissipation that occurs even under dark storage conditions.
There remains a need for a method and apparatus for solving the problem of hologram erasure that occurs during readout of holograms, as well as in storage conditions. Commercial memory storage devices require that a hologram should be written into a photorefractive material, fixed for permanent storage, and selectively erased if erasure is desired. It is also necessary for most applications that the stored image must demonstrate no loss in quality after multiple readouts, not demonstrate appreciable dark storage degradation over the lifetime of the device, and not demonstrate appreciable degradation as the image is read-out. The method and apparatus should be practicable without expensive optical components, such as a polychromatic polarized laser light source or rotating polarizing filters.
The present invention overcomes the problems outlined above by providing method and apparatus for implementing an all-optical and reversible technique that solves the problem of hologram erasure or fading in photorefractive materials both during image readout and during storage conditions. Holograms are permanently stored in photorefractive materials by using incoherent ultraviolet light to fix the holograms for indefinite preservation and storage in the photorefractive material. The stored holograms may be read out non-destructively and erased, as desired. The technique is a major breakthrough that advances volume storage in photorefractive materials toward commercial systems having practical and economical applications.
It has been discovered that holograms are fixed in a doped or undoped photorefractive material when the photorefractive material is irradiated with fixing radiation from the ultraviolet light, gamma radiation, or X-ray spectra. Ultraviolet light is particularly preferred, and polychromatic ultraviolet radiation having a wavelength from 200 to 400 nm is especially preferred. While ultraviolet light has traditionally been used to erase holograms, a lower level of exposure to ultraviolet light is now shown to fix the images for permanent storage that resists image fading during readout or during storage conditions.
The process is an all optical process where the fixing radiation can be coherent or incoherent light. Ultraviolet light refers to light radiation that is made up of a collection of wavelengths in the ultraviolet light spectrum, and is preferably a subportion of this spectrum, such as deep UV, that is tuned to match the sensitivity of a particular photorefractive material. The information contained in the holograms is frozen indefinitely in the material, and is stable against uniform illumination of the material. The fixed holograms are also erased by further irradiation with ultraviolet light thus making the process reversible. The ultraviolet light that is used for both the image fixing process and the erasure process is preferably incoherent ultraviolet light.
The new fixation technique is considerably less expensive than prior fixation techniques. Additionally, the electro-optical properties of a light sensitive material are changed when it is irradiated with ultraviolet light. These new materials can be substituted for known materials of similar composition in devices including gyroscopes and optical switches in telecommunications systems where the photorefractive material is stabilized against imprint. The change in electro-optical properties can also be used to enhance the recording and erasure characteristics of a photorefractive material, and stronger light can advantageously be used to readout images without image degradation.
Aspects of the present invention include:
a. providing a reversible technique to fix holograms in a photorefractive material for long term storage and erase them optically;
b. providing a repeatable and robust method that can be used to fix holograms in a doped or undoped material and erase them;
c. providing a simple and practical method to control the state of information stored in photorefractive materials;
d. providing ways to design compact high-capacity volume memories;
e. providing ways to advance this technology toward commercial compact storage systems;
f. providing a method for designing archival systems;
g. providing means to develop economical high-capacity volume memories since the process detailed in this invention is used with both incoherent light and incoherent light;
h. providing a method to preserve information in photorefractive materials without the use of complex and expensive hardware;
i. providing the option to design archival read only and read and write information storage systems;
j. providing novel ways to further understand light interaction in photorefractive materials so that their properties can be optimized for various optical processing applications;
k. providing ways to alter the light properties of a light sensitive material;
l. providing a technique to enhance the recording and erasure characteristics of photorefractive materials; and
m. providing new materials having substantially improved electro-optical properties for optical image and information processing systems, as well as for other electro-optical systems including at least gyroscopes and optical switches.
Further objects and advantages are to provide a universal technique to fix information permanently in different doped or undoped photorefractive materials. Moreover, this invention will pave the road for material developers to devise novel materials for optical processing applications. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.